1. Field of the Invention
The present invention relates to a scanning electron microscope and, more particularly, to a scanning electron microscope which displays a grayscale image.
2. Description of the Related Art
In the manufacturing process of a semiconductor, in order to increase a yield, defects must be inspected, and causes of the defects must be investigated. In inspection of defects, an observing apparatus such as a scanning electron microscope or optical microscope is used.
In the scanning electron microscope, scattered electrons and backscattered electrons are generated by irradiating a primary electron beam on a sample to obtain a scattered electron image and a backscattered electron image.
The scattered electron image corresponds to an image obtained when illumination light is irradiated from the same direction as that of an observing direction to observe a sample. Therefore, a sharp pattern edge, a change of materials, and the like are displayed. However, microscopic unevenness cannot be clearly displayed. A backscattered electron image corresponds to an image obtained when illumination light is irradiated from a direction oblique to an observing direction to observe a sample. Therefore, since a grayscale profile image can be obtained, a microscopic unevenness can be clearly displayed.
Defects of a semiconductor include various types of defects. As typical defects, a foreign matter or contamination caused by residual of chemicals, pattern cracking, scratches generated in a polishing step, unevenness in a preprocess, and the like are known.
A microscopic unevenness such as a scratch generally has a low S/N ratio, so that the unevenness can be imaged as a backscattered electron image rather than a scattered electron image. However, a very microscopic scratch cannot obtain a sufficient S/N ratio even though a backscattered electron image is used, a clear image may not be obtained.
Methods of clearly picking up an image of the microscopic unevenness are described in the following Patent Documents. In these methods, a plurality of detectors is used.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-116161
[Patent Document 2] U.S. Pat. No. 4,941,980
[Patent Document 3] Japanese Patent Application Laid-Open No. 2000-173526
In the examples described in the above Patent Documents, a plurality of detectors is used. However, when an unevenness of a sample is microscopic, the S/N ratios of signals detected by the detectors become low. Therefore, use of the plurality of detectors does not make it possible to clearly detect the microscopic unevenness.